Electrolytic Capacitor Having A Tantalum Anode

ABSTRACT

A wet tantalum electrolytic capacitor containing a cathode, fluidic working electrolyte, and anode formed from an anodically oxidized sintered porous tantalum pellet is provided. The pellet is formed from a pressed tantalum powder. The tantalum powder is formed by reacting a tantalum oxide compound, for example, tantalum pentoxide, with a reducing agent that contains a metal having an oxidation state of 2 or more, for example, magnesium. The resulting tantalum powder is nodular or angular and has a specific charge that ranges from about 11,000 μF*V/g to about 14,000 μF*V/g. Using this powder, wet tantalum electrolytic capacitors have breakdown voltages that ranges from about 250 volts to about 400 volts. This makes the electrolytic capacitors ideal for use in an implantable medical device.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional patent applicationSer. No. 63/004,532, filed on Apr. 3, 2020.

BACKGROUND OF THE INVENTION 1. Field of the Invention

High voltage electrolytic capacitors are often employed in implantablemedical devices. However, because it is desirable to minimize theoverall size of the implanted device, capacitors that are suitable forhigh voltage implantable applications are required to have a relativelyhigh energy density. This is particularly the case for an implantablecardioverter defibrillator (“ICD”), also referred to as an implantabledefibrillator, because the high voltage capacitors used to deliver thedefibrillation pulse can occupy as much as one third of the ICD volume.ICDs typically use two to four electrolytic capacitors in series toachieve the desired high voltage for shock delivery.

One commonly used high voltage capacitor is referred to as a wettantalum electrolytic capacitor. A wet tantalum capacitor has an anodeof a porous sintered tantalum pellet. For example, a tantalum pellet maybe formed by compressing a tantalum powder under high pressure followedby sintering at high temperature to form a sponge-like structure, whichis very strong and dense but also highly porous. Because of the highvoltages encountered in medical devices, however, relatively lowspecific charge tantalum powders must be employed. Namely, if thespecific charge is too high, relatively thin sinter necks tend to formbetween adjacent particles, which can cause the dielectric layer at andadjacent to the sinter necks to fail at high voltages.

2. Prior Art

U.S. Pat. No. 10,290,430 to Djebara et al. relates to a tantalummaterial for use as an anode in a wet tantalum electrolytic capacitor.The tantalum powder has a specific charge that ranges from about 15,000μF*V/g to about 45,000 μF*V/g, which results in a capacitor that canoperate at a voltage that ranges from about 100 volts to about 300volts. This patent further describes that the tantalum material haslarger sinter necks that result in higher charge voltages.

However, to further improve the energy density of an electrolyticcapacitor for use in an implantable medical device, a need currentlyexists for an improved tantalum powder with a lower specific capacitancethan the tantalum material described in the '430 patent. The resultingimproved electrolytic capacitor is intended for use in an implantablemedical device, such as an implantable defibrillator.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a wettantalum electrolytic capacitor is disclosed that comprises an anode, acathode, and a fluidic working electrolyte in contact with the anode andthe cathode. The anode comprises an anodically oxidized tantalum pelletformed from a pressed and sintered tantalum powder. The tantalum powderis formed by reacting an oxide of a tantalum compound with a reducingagent that contains a metal having an oxidation state of 2 or more. Thecathode comprises a metal substrate coated with a pseudocapacitivecoating, preferably of ruthenium oxide coating.

In accordance with another embodiment of the present invention, a wettantalum electrolytic capacitor is disclosed that comprises an anode, acathode, and a fluidic working electrolyte in contact with the anode andcathode. The anode comprises an anodically oxidized tantalum pelletformed from a pressed and sintered tantalum powder. The tantalum powderis nodular or angular and has a specific charge that ranges from about11,000 μF*V/g to about 14,000 μF*V/g. The cathode comprises a metalsubstrate coated with a pseudocapacitive coating, preferably ofruthenium oxide coating.

In accordance with yet another embodiment of the present invention, amethod for forming a wet tantalum electrolytic capacitor is disclosedthat comprises pressing a tantalum powder into a pellet. The tantalumpowder is formed by reacting tantalum pentoxide with a reducing agentthat contains magnesium, calcium, strontium, barium, cesium, aluminum,or a combination thereof, sintering the pellet, anodically oxidizing thesintered pellet to form a dielectric layer that overlies the tantalumpowder, and positioning the anode in electrical association with acathode through contact with a fluidic working electrolyte in a casing.

These and other aspects of the present invention will become moreapparent to those skilled in the art by reference to the followingdescription and to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary capacitor 10 according tothe present invention.

FIG. 2 is a partial cross-sectional view of the capacitor 10 shown inFIG. 1 but comprising a casing 30 of mating clamshell-type members 38,46 housing a single anode 54 operatively associated with a cathode 56supported on the inner surfaces of the casing members.

FIG. 3 is a side elevation view of an anode assembly comprising a pairof anodes 58 and 60 connected in parallel to the anode terminal wire 64of a feedthrough 66.

FIG. 4 is a side elevational view of the anodes 58 and 60 shown in FIG.3 for use in the capacitor 10 shown in FIG. 1.

FIG. 5 is a plan view of the top edge of the anodes 58 and 60 shown inFIG. 4.

FIG. 6 is a perspective view of the anode assembly shown in FIG. 3 withan intermediate cathode current collector 74 being disposed between thepair of anodes 58 and 60.

FIG. 7 is a photograph of a tantalum powder according to the presentinvention at 4,000× magnification.

FIG. 8 is a graph of the energy density of tantalum powder lots A and Baccording to the example described herein.

FIG. 9 is a graph of the leakage current (DCL) of various tantalum powerlots A and B according to the example described herein.

FIG. 10 is a graph plotting DCL vs. J/cc for two samples of tantalumpowder lots A and B according to the example described herein.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

FIG. 1 is a perspective view of an exemplary wet tantalum electrolyticcapacitor 10 according to the present invention. The capacitor 10comprises at least one tantalum anode and a cathode of a cathodematerial housed inside a hermetically sealed casing 12. The illustratedcasing 12 is exemplary of any one of a myriad of shapes for a capacitor,limited only by the device that the capacitor 10 is designed to be usedin. The tantalum anode and the cathode housed inside the casing 12 arein electrical association with each other by a working electrolyte (notshown) contained therein. The anode or anodes, cathode, and workingelectrolyte of the capacitor 10 will be described in detail hereinafter.

The exemplary casing 12 shown in FIG. 1 has a first drawn casing member14 closed by a lid 16. The casing members 14, 16 are preferably selectedfrom the group of titanium, tantalum, nickel, molybdenum, niobium,cobalt, stainless steel, tungsten, platinum, palladium, silver, copper,chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron, andmixtures and alloys thereof. In addition to being of a drawn form,casing member 14 can be of a machined construction or be formed by ametal injection molding process. Preferably, the casing members have athickness of about 0.001 inches to about 0.015 inches.

The first casing member 14 has a first face wall 18 joined to asurrounding side wall 20 extending to an edge 22. The second casingmember 16 has the shape of a plate with a second face wall 24 having asurrounding edge 26. The casing members 14 and 16 are hermeticallysealed together by welding the overlapping or abutting edges 22 and 26where they contact each other. The weld is preferably provided by laserwelding.

FIG. 2 illustrates another embodiment of an exemplary casing 30according to the present invention. The exemplary casing 30 has matingdrawn clamshell-type member 32 and 34. The first clamshell-type casingmember 32 comprises a surrounding sidewall 36 extending to and meetingwith a major face wall 38 at a curved intermediate bend 40. Oppositebend 40, the surrounding sidewall 36 extends to a continuous, perimeteredge 42. Similarly, the second clamshell-type casing member 34 comprisesa surrounding sidewall 44 extending to and meeting with a major facewall 46 at a curved intermediate bend 48. Opposite bend 48, thesurrounding sidewall 44 extends to a continuous perimeter edge 50.However, face wall 38 is somewhat smaller than face wall 46 so that itssurrounding sidewall 36 fits inside the surrounding sidewall 44 of thesecond casing member 34 in an overlapping, contact relationship. Thatway, casing 30 is hermetically sealed by providing a weld 52 at thesurrounding sidewall 36 of the first casing member 32 and the edge 50 ofthe second casing member 34.

Other casing structures that are useful with the present capacitor 10are described in U.S. Pat. No. 7,012,799 to Muffoletto et al., U.S. Pat.No. 7,092,242 to Gloss et al., U.S. Pat. No. 7,271,994 to Stemen et al.,U.S. Pat. No. 9,978,528 to Hahl et al., U.S. Pat. No. 9,721,730 toMuffoletto et al., U.S. Pat. No. 9,824,829 to Muffoletto et al., U.S.Pat. No. 9,875,855 to Perez et al. and U.S. Pat. No. 10,020,127 toMuffoletto, all of which are assigned to the assignee of the presentinvention and incorporated herein by reference.

As shown in FIG. 2, a weld strap 53 is provided directly adjacent to thesurrounding sidewall 36 of casing member 32. The weld strap 53 is anannular, ring-shaped member surrounding the anode end wall 54C and istypically of the same metal as that of the casing members 32, 46. Anintermediate polymeric insulating ring 55 seats against the weld strap53 and a separator 68 portion covering the anode end wall 54C. The weldstrap 53 in conjunction with the insulating ring 55 help shield theseparator 68 at the anode end wall 54C from the heat generated as thecasing members 32, 46 are welded together.

The single anode 54 housed inside the exemplary casing 30 illustrated inFIG. 2 is of tantalum. As is well known by those skilled in the art, theanode metal in powdered form, for example tantalum powder, is compressedinto a pellet of a desired shape. In the illustrated embodiment, theanode pellet is of a substantially uniform thickness extending to spacedapart right and left major sidewalls 54A, 54B joined by an end wall 54C.The major sidewalls 54A, 54B meet the intermediate wall 54C atrespective curved edges 54D, 54E. The curved edges 54D, 54E are of asubstantially similar radius as that of the casing bends 40, 48,respectively.

The cathode material 56 preferably coats the inner surfaces of the majorface walls 38, 46 of the respective casing members 32, 34 in a patternthat generally mirrors the shape of the right and left major sidewalls54A, 54B of the anode 54. The cathode material 56 preferably has athickness of about a few hundred Angstroms to about 0.1 millimeters andis either directly coated on the inner surfaces of the face walls 38, 46or it is coated on a conductive substrate (not shown) supported on andin electrical contact with the inner surfaces thereof. The cathodematerial coatings are preferably spaced from the surrounding sidewalls36, 44 of the respective casing members 32, 34.

In that respect, the major face walls 38, 46 of the casing members 32,34 may be of an anodized-etched conductive material, have a sinteredactive material with or without oxide contacted thereto, be contactedwith a double layer capacitive material, for example a finely dividedcarbonaceous material such as graphite, carbon, platinum black, a redox,pseudocapacitive, or an under potential material, or be an electroactiveconducting polymer such as polyaniline, polypyrrole, polythiophene,polyacetylene, and mixtures thereof.

According to one preferred aspect of the present invention, the redox orcathode material 56 includes an oxide of a metal, a nitride of themetal, a carbon nitride of the metal, and/or a carbide of the metal, theoxide, nitride, carbon nitride and carbide having pseudocapacitiveproperties. The metal is preferably selected from the group consistingof ruthenium, cobalt, manganese, molybdenum, tungsten, tantalum, iron,niobium, iridium, titanium, zirconium, hafnium, rhodium, vanadium,osmium, palladium, platinum, nickel, and lead. In a preferred embodimentof the present invention, the cathode material 56 includes an oxide ofruthenium or oxides of ruthenium and tantalum.

A pad printing process as described in U.S. Pat. No. 7,116,547 to Seitzet al. is preferred for providing such a coating. An ultrasonicallygenerated aerosol as described in U.S. Pat. Nos. 5,894,403, 5,920,455,6,224,985, and 6,468,605, all to Shah et al., is also a suitabledeposition method. These patents are assigned to the assignee of thepresent invention and incorporated herein by reference.

In contrast to the single anode embodiment shown in the exemplarycapacitor of FIGS. 1 and 2, FIG. 3 is a side elevation view of an anodeassembly for the capacitor 10 of FIG. 1 where the anode assemblyincluding a first anode 58 and a second anode 60. For the sake ofclarity of illustration, the anode assembly is depicted prior to bendingthe connecting wire so that the faces of the anodes 58 and 60 arepositioned near or adjacent to each other with a cathode currentcollector supporting a cathode material disposed there between.

The first anode 58 comprises an outer major face wall 58A opposite aninner major face wall (not numbered), both face walls extending to asurrounding edge 58B. Similarly, the second anode 60 comprises an outermajor face wall 60A opposite an inner major face wall (not numbered),both face walls extending to a surrounding edge 60B. A connecting wire62 has a first portion 62A embedded in the first anode 58 and a secondportion 62B embedded in the second anode 60. The connecting wire 62 is acontinuous member that is electrically connected to the J-shapedinterior portion 64A of an anode terminal wire 64 of a hermeticfeedthrough 66 by a suitable joining process, such as laser welding.This means that the first anode 58 and the second anode 60 are connectedto the terminal wire 64 in parallel. Alternatively, the first wireportion 62A and the second wire portion 62B are separate wires (notshown) that are joined to the J-shaped portion 64A of the terminal wire64.

While the casing 12 shown in FIG. 1 is described as housing a dual anodesystem 58 and 60 and the mating clamshell-type casing 30 shown in FIG. 2is described as housing a single anode 54, it is within the scope of thepresent invention that either casing 12 or 30 can house a single anode,two anodes, or more than two anodes. If more than two anodes, forexample “n” anodes, there will be n−1 cathode current collectorssupporting a cathode material positioned between immediately adjacentanodes.

As shown in FIG. 4, in an exemplary embodiment, the length “L” of thetantalum anodes 54 (FIG. 2) and 58 and 60 (FIG. 3) ranges from about 1to about 80 millimeters, more particularly from about 10 to about 60millimeters, and still more particularly from about 20 to about 50millimeters. Likewise, the length “L₁” that the wires 62A, 62B areembedded in the tantalum anode 54, 58 and 60 preferably ranges fromabout 1 to about 40 millimeters, in some embodiments, from about 2 toabout 20 millimeters, and in some embodiments, from about 5 to about 15millimeters. The width “W” of the tantalum anodes 54, 58 and 60 ispreferably from about 0.05 to about 40 millimeters, in some embodiments,from about 0.5 to about 35 millimeters, and in some embodiments, fromabout 2 to about 25 millimeters.

To improve the electrical performance and volumetric efficiency of thecapacitor 10, the thickness of the anodes 54, 58 and 60 is relativelythin. For example, the thickness of the anodes 54, 58 and 60 representedby the dimension “H” in FIG. 5 is about 5 millimeters or less, moreparticularly from about 0.05 to about 4 millimeters, and still moreparticularly from about 0.1 to about 3.5 millimeters. The ratio of thelength of the anode to its thickness is from about 5 to about 50, moreparticularly from about 6 to about 30, and still more particularly fromabout 7 to about 20. Although shown having a “D-shape” in FIGS. 3 to 5,the anodes 54, 58 and 60 may have any desired shape, such as square,rectangle, circle, oval, triangle, and the like.

In that respect, the anodes 54, 58 and 60 are individually formed froman anodically oxidized sintered porous valve metal pellet, preferably atantalum powder pellet. Preferably, each anode pellet is made from apressed tantalum powder, which is formed by reacting a tantalum oxide,for example, tantalum pentoxide, with a reducing agent that contains ametal having a relatively high oxidation state, for example, anoxidation state of 2 or more. Exemplary metals for the reducing agentinclude alkaline earth metals, for example, magnesium, calcium,strontium, barium, cesium, aluminum, and the like. Through use of suchtantalum powder, an electrolytic capacitor exhibiting relatively highcapacitance is achieved, which is advantageous when the intended use ofthe capacitor 10 is for use in an implantable medical device. Thetantalum powder is preferably a tantalum oxide that is capable of beingreduced, such as Ta₂O_(x) (x≤5), for example, Ta₂O₅.

The reducing agent is provided in a gaseous, liquid, or solid state, andmay be in the form of the metal, as well as alloys or salts thereof. Inone embodiment, a halide salt, for example, a chloride or fluoride isused. If desired, other components may be added before, during, or afterthe reduction reaction, such as dopants, alkali metals, and the like.

Reduction of the tantalum oxide is typically carried out at atemperature that ranges from about 400° C. to about 1,200° C., and insome embodiments from about 600° C. to about 1,000° C., for about 20 toabout 300 minutes. Heating may be carried out in a reactor under aninert atmosphere, for example, an argon or nitrogen atmosphere so that amolten bath is formed. Suitable reactors include vertical tube furnaces,rotary kilns, fluid bed furnaces, multiple hearth furnaces,self-propagation high-temperature synthesis reactors, and the like.Preferably the reactor is maintained under an inert gas until the massin the reaction vessel is cooled to ambient temperature. Additionaldetails of such a reduction reaction are described in U.S. Pub. Nos.2003/0110890 to He et al. and 2004/0163491 to Shekhter et al.

After the reduction reaction, the tantalum powder is cooled, crushed,and washed to remove excess impurities or reactants. The washingsolution may include, for example, a mineral acid and water. If desired,the tantalum powder may be subjected to additional treatment to removeany tantalates, for example, magnesium tantalate that may have formedduring the reduction reaction. For example, one technique for removingtantalates involves heating the tantalum powder under vacuum at atemperature that ranges from about 1,100° C. to about 1,400° C. forabout 15 minutes to about 6 hours. Another technique for removingtantalates involves heating the tantalum powder at a temperature thatranges from about 800° C. to about 1,300° C. in the presence of a gettermaterial, such as magnesium, calcium and/or aluminum, for about 15minutes to about 6 hours. Such techniques are described in more detailin U.S. Pat. No. 7,431,751 to Shekhter et al. Although not required, theresulting tantalum powder may be subjected to additional refining stepsas is known in the art, such as doping, deoxidizing, and the like.

Regardless the method used, the resulting tantalum powder is afree-flowing, finely divided powder that contains primary particleshaving a three-dimensional shape, such as a nodular or angular shape.Such particles are not generally flat and thus have a relatively low“aspect ratio”, which is defined as the average diameter or width of theparticles divided by the average thickness (“D/T”). For example, theaspect ratio of the tantalum particles may be about 4 or less, in otherembodiments about 3 or less, and in still other embodiments from about 1to about 2.

The tantalum powder also has a relatively high specific surface area,such as about 0.5 square meter per gram (“m²/g”) or more, in otherembodiments about 2 m²/g or more. The term “specific surface area”generally refers to surface area as determined by the physical gasadsorption (B.E.T.) method described by Bruanauer, Emmet, and Teller,Journal of American Chemical Society, Vol. 60, 1938, p. 309, withnitrogen as the adsorption gas. The test may be conducted with aMONOSORB® Specific Surface Area Analyzer available from QuantachromeCorporation, Syosset, N.Y., which measures the quantity of adsorbatenitrogen gas adsorbed on a solid surface by sensing the change inthermal conductivity of a flowing mixture of adsorbate and inert carriergas, for example, helium.

Due to its relatively high surface area and low particle size, thetantalum powder of the present invention has a high specific charge thatranges from about 11,000 to 14,000 microFarads*Volts per gram(“μF*V/g”). As is known in the art, the specific charge is determined bymultiplying capacitance by the anodizing voltage employed, and thendividing this product by the weight of the anodized tantalum body.Despite the use of such high specific charge tantalum powders withthree-dimensional particles, the reduction process described above isbelieved to result in “sinter necks” between adjacent agglomeratedparticles that are relatively large. Sinter necks are the smallcross-sectional area of the electrical path within the metal structure.Typically, the sinter necks have a size of about 200 nanometers, ormore, in some embodiments about 250 nanometers, or more, and in someembodiments, from about 300 to about 800 nanometers. Because the sinternecks are relatively large, the dielectric layer near the neck is morelikely not to fail at high forming voltages.

The tantalum powder (as well as the exemplary anodes 54, 58 and 60formed from the powder) has a relatively low alkali metal, carbon, andoxygen content. For example, the tantalum powder has no more than about50 ppm carbon or alkali metals, and in some embodiments, no more thanabout 10 ppm carbon or alkali metals. More particularly, two batches oftantalum powder received from H. C. Starck, now Taniobis GmbH, wereanalyzed. The qualitative results are listed below.

Tantalum Powder Lots Characteristic H 22 33 N 293 261 O 1456 1255 Mg 7 7C 32 32 Fe <3 <3 Cr <2 <2 Ni <3 <3 Si <3 <3 P 21 21 Apparent density,Scott (cube) g/cm³ 1.8 2.1 Flow time 0.15 inch/Vib.40/25 g (sec) 20 16Specific surface TriStar m²/g 0.5 0.6 Pellet Weight (mg) 150 150 PelletShape Cylindrical Cylindrical Pellet Length (mm) 3.8 3.7 Pellet Diameter(mm) 3.0 3.0 Pellet Density (g/cm³) 5.4 5.4 Sintering Temperature (° C.)1500 1560 Sintering Time (min) 20 20 Shrinkage, Volume (%) 10.7 11.6Formation Voltage (V) 300 300 Formation Condition H3PO4/Glycol (uS/cm)350 350 Formation Temperature (° C.) 60 60 Capacitance @ 120 Hz (μFV/g)12940 12052 Leakage Current (nA/μFV) 1.9 2.4

To facilitate construction of the anodes 54, 58 and 60, additionalmaterials may be included in the tantalum powder. For example, thetantalum powder may be mixed with a binder and/or a lubricant to ensurethat the particles adequately adhere to each other when compacted orpressed to form a pellet. Suitable binders include, for example,poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol);poly(vinyl pyrollidone); cellulosic polymers, such ascarboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, and methylhydroxyethyl cellulose; atactic polypropylene;polyethylene; polyethylene glycol (e.g., Carbowax from Dow ChemicalCo.); polystyrene, poly(butadiene/styrene); polyamides; polyimides; andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, micro-waxes(purified paraffins), and the like.

The binder may be dissolved or dispersed in a lubricating solvent.Exemplary solvents include water, alcohols, and the like. When utilized,the percentage of binder and/or lubricant may range from about 0.1% toabout 8% by weight of the total mass. It should be understood, however,that a binder and a lubricant is not necessarily required to form theexemplary anodes 54, 58 and 60 for use in the capacitor 10.

To form an anode, the resulting tantalum powder is compacted into apellet using any conventional powder press device. For example, a pressmold may be employed that is a single station compaction presscontaining a die and one or multiple punches. Alternatively, ananvil-type compaction press mold that uses only a die and single lowerpunch may be used. Single station compaction press molds are availablein several basic types, such as cam, toggle/knuckle and eccentric/crankpresses with varying capabilities, such as single action, double action,floating die, movable platen, opposed ram, screw, impact, hot pressing,coining or sizing. As previously shown in FIG. 3, the tantalum powdermay be compacted around the connecting wire 62, 62A and 62B that isformed from any electrically conductive material, such as tantalum,niobium, aluminum, hafnium, titanium, and the like, as well aselectrically conductive oxides and/or nitrides thereof. Preferably, atantalum connecting wire 62 is used with a tantalum powder pellet.

If a binder or lubricating solvent is used, the binder or lubricant isremoved after pressing by heating the pellet under vacuum at atemperature that ranges from about 150° C. to about 500° C. for severalminutes. Alternatively, the binder/lubricant is removed by contactingthe tantalum pellet with an aqueous solution, such as described in U.S.Pat. No. 6,197,252 to Bishop et al.

The tantalum pellet is then sintered to form a porous, integral mass.Sintering typically occurs at a temperature that ranges from about 800°C. to about 2,000° C., more particularly from about 1,200° C. to about1,800°, and still more particularly from about 1,500° C. to about 1,700°C. Suitable sintering times range from about 5 minutes to about 100minutes, and more particularly from about 8 minutes to about 15 minutes.

The sintering heating profile typically occurs in one or more steps.Preferably, sintering occurs in an atmosphere that limits the transferof oxygen atoms to the tantalum. For example, sintering may occur in areducing atmosphere, such as in a vacuum, inert gas, hydrogen, and thelike. The reducing atmosphere may be at a pressure that ranges fromabout 10 Torr to about 2,000 Torr, more particularly from about 100 Torrto about 1,000 Torr, and still more particularly from about 100 Torr toabout 930 Torr. Mixtures of hydrogen and other gases, for example, argonor nitrogen may also be employed as the sintering atmosphere.

Sintering causes the tantalum pellet to shrink due to the growth ofmetallurgical bonds between the tantalum particles. Because shrinkagegenerally increases the density of the pellet, lower press densities(“green”) may be employed to achieve the desired target density. Forexample, the target density of the anode pellet after sinteringtypically ranges from about 5 grams per cubic centimeter (g/cm³) toabout 8 g/cm³. Because of the shrinking phenomenon, however, thetantalum pellet need not be pressed to such high densities but mayinstead be pressed to densities of less than about 6.0 g/cm³, and moreparticularly from about 4.5 g/cm³ to about 5.5 g/cm³.

The exemplary anodes 54, 58 and 60 contain a dielectric formed byanodically oxidizing (“anodizing”) a sintered pellet so that adielectric layer is formed over and within the tantalum forming thepellet. For example, the tantalum anodes 54, 58 and 60 may be anodizedto form tantalum pentoxide (Ta₂O₅). Typically, anodization is performedby positioning the tantalum pellet in an anodizing electrolyte. Aqueoussolvents (e.g., water) and/or non-aqueous solvents, for example,ethylene glycol may be employed in the anodizing electrolyte.

To enhance conductivity of the anodizing electrolyte, a compound may beused that is capable of dissociating in the solvent to form ions.Examples of such compounds include, for example, acids, such as thosedescribed below with respect to the working electrolyte. For example,phosphoric acid may constitute from about 0.01 wt. % to about 5 wt. %,particularly from about 0.05 wt. % to about 0.8 wt. %, and still moreparticularly from about 0.1 wt. % to about 0.5 wt. % of the anodizingelectrolyte. If desired, blends of acids may also be employed.

During the anodizing process, a current is passed through the anodizingelectrolyte to form the dielectric layer on the tantalum particles. Thevalue of the formation voltage determines the thickness of thedielectric layer. For example, the power supply may be initially set ata galvanostatic mode until the required voltage is reached. Thereafter,the power supply may be switched to a potentiostatic mode to ensure thatthe desired dielectric thickness is formed over the entire surface ofthe tantalum pellet. Of course, other known anodizing methods may beemployed, such as pulse or step potentiostatic methods.

To achieve a capacitor capable of operating at a relatively high voltagerange, the voltage at which anodic oxidation occurs is relatively high.The anodizing voltage typically ranges from about 100 volts to about 500volts, more particularly from about 200 volts to about 450 volts, andstill more particularly from about 250 volts to about 430 volts. Thetemperature of the anodizing electrolyte ranges from about 10° C. toabout 200° C., more particularly from about 20° C. to about 150° C., andstill more particularly from about 30° C. to about 90° C. The resultingdielectric layer is formed on both the outer surface of the tantalumpellet and within pores inside the pellet.

U.S. Pat. No. 6,231,993 to Stephenson et al. describes an anodizingprocess where a tantalum pellet is formed by periodically holding thepellet at a constant voltage and allowing the current to decay over aperiod, or by turning the formation power supply off altogether duringthe anodization process. Either way provides an opportunity for heatedanodizing electrolyte to diffuse from the anodized tantalum pellet. The'993 patent is assigned to the assignee of the present invention andincorporated herein by reference.

The capacitor 10 further comprises separators of electrically insulativematerial that surround and envelop the anodes 54, 58 and 60. As shown inFIG. 2, a separator envelope 68 encloses the single anode 54. FIG. 6shows a first separator envelope 70 encloses the first anode 58 and asecond separator envelope 72 encloses the second anode 60 in the dualanode assembly. In that manner, the respective separators 68, 70 and 72prevent an internal electrical short circuit between the anode andcathode materials in the assembled capacitor 10 and have a degree ofporosity sufficient to allow flow therethrough of the workingelectrolyte during the electrochemical reaction of the capacitor.Illustrative separator materials include woven and non-woven fabrics ofpolyolefinic fibers including polypropylene and polyethylene, orfluoropolymeric fibers including polyvinylidene fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylenelaminated or superposed with a polyolefinic or fluoropolymericmicroporous film, non-woven glass, glass fiber materials and ceramicmaterials.

Suitable microporous films include a polyethylene membrane commerciallyavailable under the designation SOLUPOR®, (DMS Solutech); apolytetrafluoroethylene membrane commercially available under thedesignation ZITEX®, (Chemplast Inc.) or EXCELLERATOR®, (W. L. Gore andAssociates); a polypropylene membrane commercially available under thedesignation CELGARD®, (Celgard LLC); and a membrane commerciallyavailable under the designation DEXIGLAS®, (C. H. Dexter, Div., DexterCorp.). Cellulose based separators also typically used in capacitors arecontemplated by the scope of the present invention. Depending on theelectrolyte used, the separator can be treated to improve itswettability, for example with a surfactant, as is well known by thoseskilled in the art.

The cathode of capacitor 10 comprises a cathode material supported byand in contact with the inner surfaces of face walls of the casingmembers. In the casing 12 shown in FIG. 1, the cathode material issupported on the face walls 18 and 24 of the respective casing members14 and 16. In the embodiment shown in FIG. 2, the cathode material issupported by and in contact with the inner surfaces of the face walls 38and 46 of the casing members 32 and 34. In any event, the cathodematerial has a thickness of about a few hundred Angstroms to about 0.1millimeters directly coated on the inner surfaces of the face walls ofcasing members, or the cathode material may be coated on a conductivesubstrate (not shown) in electrical contact with the inner surface ofthe face walls. With reference to the dual anode embodiment shown inFIGS. 3 and 6, cathode material 68 is also positioned intermediate theanodes 58 and 60, supported on the opposed first and second major faces74A and 74B of a cathode current collector 74, which is preferably inthe form of a foil. In that respect, the respective face walls 14, 16(FIG. 1) and 38, 46 (FIG. 2) and the intermediate current collector 74(FIG. 6) may be of an anodized-etched conductive material, or have asintered active material with or without oxide contacted thereto, or becontacted with a double layer capacitive material, for example a finelydivided carbonaceous material such as graphite or carbon or platinumblack, or be contacted with a redox, pseudocapacitive or an underpotential material, or an electroactive conducting polymer such aspolyaniline, polypyrrole, polythiophene, and polyacetylene, and mixturesthereof.

According to one preferred aspect of the present capacitor 10, the redoxor cathode material 68 includes an oxide of a metal, the nitride of themetal, the carbon nitride of the metal, and/or the carbide of the metal,the oxide, nitride, carbon nitride and carbide having pseudocapacitiveproperties. The metal is preferably selected from the group consistingof ruthenium, cobalt, manganese, molybdenum, tungsten, tantalum, iron,niobium, iridium, titanium, zirconium, hafnium, rhodium, vanadium,osmium, palladium, platinum, nickel, and lead. Ruthenium oxide is apreferred cathode material 68 coated on the interior surfaces of therespective casing members 14, 16 (FIG. 1) and 38, 46 (FIG. 2) and on theintermediate current collector (FIG. 6) or a separate conductivesubstrate is electrically supported on the interior surfaces of thecasing members.

FIG. 6 shows that although the wire 62 connecting the anode 58 and 60 inparallel is potted in a polymer block 76, the anodes are slightlyseparable due to the flexibility of the embedded wire at the junction ofthe polymer and the anode pellets themselves. The anodes 58 and 60 aresufficiently separable to allow the cathode current collector 74 to beinserted between them, as indicated by arrows 78. That way, the cathodecurrent collector 74 is positioned between the opposed inner faces ofthe first and second anodes 58 and 60 with the first and second majorfaces 74A and 74B of the cathode current collector having the cathodematerial 68 supported thereon facing the anodes, thereby forming ananode-cathode assembly. A tab 80 that extends outwardly from the cathodecurrent collector 74 is configured for tack welding to the inner surfaceof the surrounding side wall 20 of casing member 14. The tab 80 is bentapproximately perpendicular to the faces 74A and 74B to position it forwelding to side wall 20.

In a functioning capacitor 10, a working electrolyte is in electricalcontact or communication with the cathode material 68 and the anodes 54,58 and 60. The working electrolyte is a fluid that may soaked into theanodes 54, 58 and 60, or it may be added to the capacitor 10 through afill port 82 (FIG. 1) at a later stage of production. Preferably, thefluidic electrolyte uniformly wets the dielectric on the tantalum anodes54, 58 and 60.

Typically, the working electrolyte has an electrical conductivity offrom about 0.01 Siemens per centimeter (“S/cm”) to about 0.1 S/cm, moreparticularly from about 0.02 S/cm to about 15 S/cm, and still moreparticularly from about 0.02 S/cm to about 10 S/cm, determined at atemperature of about 25° C. using any known electric conductivity meter.The working electrolyte is generally in the form of a liquid, such as anaqueous or non-aqueous solution. For example, the electrolyte may be anaqueous solution of an acid (e.g., sulfuric acid, phosphoric acid, ornitric acid), base (e.g., potassium hydroxide), or salt (e.g., ammoniumsalt, such as a nitrate), as well any other suitable electrolyte knownin the art, such as a salt dissolved in an organic solvent (e.g.,ammonium salt dissolved in a glycol-based solution).

The desired ionic conductivity may be achieved by selecting ioniccompounds, for example, acids, bases, salts, and the like, withincertain concentration ranges. In one embodiment, salts of weak organicacids may be effective in achieving the desired electrolyteconductivity. The cation of the salt may include monatomic cations, suchas alkali metals (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺), alkaline earthmetals, for example, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺, transition metals,for example, Ag⁺, Fe²⁺, Fe³⁺, as well as polyatomic cations, such asNH⁴. The monovalent ammonium (NH₄ ⁺), sodium (K⁺), and lithium (Li⁺) areparticularly suitable cations for use in the present capacitor. Theorganic acid used to form the anion of the salt may be “weak” in thesense that the acid typically has a first acid dissociation constant(pK_(a1)) of about 0 to about 11, more particularly about 1 to about 10,and still more particularly from about 2 to about 10, determined atabout 23° C. Any suitable weak organic acid may be used, for example, acarboxylic acid, an acrylic acid, methacrylic acid, malonic acid,succinic acid, salicylic acid, sulfosalicylic acid, adipic acid, maleicacid, malic acid, oleic acid, gallic acid, tartaric acid (e.g.,dextotartaric acid, mesotartaric acid), citric acid, formic acid, aceticacid, glycolic acid, oxalic acid, propionic acid, phthalic acid,isophthalic acid, glutaric acid, gluconic acid, lactic acid, asparticacid, glutaminic acid, itaconic acid, trifluoroacetic acid, barbituricacid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoicacid, etc., and blends thereof. Polyprotic acids (e.g., diprotic,triprotic, etc.) are particularly desirable for use in forming the salt,such as adipic acid (pK_(a1) of 4.43 and pK_(a2) of 5.41), α-tartaricacid (pK_(a1) of 2.98 and pK_(a2) of 4.34), meso-tartaric acid (pK_(a1)of 3.22 and pK_(a2) of 4.82), oxalic acid (pK_(a1) of 1.23 and pK_(a2)of 4.19), lactic acid (pK_(a1) of 3.13, pK_(a2) of 4.76, and pK_(a3) of6.40).

While the actual amounts may vary depending on the particular saltemployed, the salt's solubility in the solvent or solvents used in theworking electrolyte, and the presence of other components, such weakorganic acid salts in the working electrolyte are in amounts that rangefrom about 0.1 wt. % to about 25 wt. %, particularly from about 0.2 wt.% to about 20 wt. %, and more particularly from about 0.3 wt. % to about15 wt. %, and still more particularly from about 0.5 wt. % to about 5wt. %.

The working electrolyte is typically aqueous in that it contains anaqueous solvent, such as deionized water. For example, deionized watermay constitute from about 20 wt. % to about 95 wt. %, more particularlyfrom about 30 wt. % to about 90 wt. %, and still more particularly fromabout 40 wt. % to about 85 wt. % of the working electrolyte.

A secondary solvent may be employed to form a solvent mixture. Suitablesecondary solvents include, for example, glycols (e.g., ethylene glycol,propylene glycol, butylene glycol, triethylene glycol, hexylene glycol,polyethylene glycols, ethoxydiglycol, dipropyleneglycol); glycol ethers(e.g., methyl glycol ether, ethyl glycol ether, isopropyl glycol ether);alcohols (e.g., methanol, ethanol, n-propanol, iso-propanol, andbutanol); ketones (e.g., acetone, methyl ethyl ketone, and methylisobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethyleneglycol ether acetate, methoxypropyl acetate, ethylene carbonate,propylene carbonate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); sulfoxides or sulfones (e.g., dimethyl sulfoxide(DMSO) and sulfolane).

Such solvent mixtures typically contain water in an amount that rangesfrom about 40 wt. % to about 80 wt. %, more particularly from about 50wt. % to about 75 wt. %, and still more particularly from about 55 wt. %to about 70 wt. %, and a secondary solvent in an amount that ranges fromabout 20 wt. % to about 60 wt. %, more particularly from about 25 wt. %to about 50 wt. %, and still more particularly from about 30 wt. % toabout 45 wt. %. The secondary solvent or solvents may, for example,constitute from about 5 wt. % to about 45 wt. %, more particularly fromabout 10 wt. % to about 40 wt. %, and still more particularly from about15 wt. % to about 35 wt. % of the working electrolyte.

If desired, the working electrolyte may be relatively neutral and have apH of from about 4.5 to about 8.0, more particularly from about 5.0 toabout 7.5, and still more particularly from about 5.5 to about 7.0. Oneor more pH adjusters, for example, acids or bases may be used to helpachieve the desired pH. In one embodiment, an acid is used to lower thepH to the desired range. Suitable acids for this purpose include, forexample, organic acids such as described above; inorganic acids, such ashydrochloric acid, nitric acid, sulfuric acid, phosphoric acid,polyphosphoric acid, boric acid, boronic acid, and mixtures thereof.Although the total concentration of the pH adjuster may vary, it istypically present in an amount that ranges from about 0.01 wt. % toabout 10 wt. %, more particularly from about 0.05 wt. % to about 5 wt.%, and still more particularly from about 0.1 wt. % to about 2 wt. % ofthe working electrolyte.

The working electrolyte may also contain other components that helpimprove the electrical performance of the capacitor 10. For instance, adepolarizer may be employed in the working electrolyte to help inhibitthe evolution of hydrogen gas at the cathode, which could otherwisecause the capacitor 10 to bulge and eventually fail. When used, thedepolarizer typically constitutes from about 1 parts per million (“ppm”)to about 500 ppm, more particularly from about 10 ppm to about 200 ppm,and still more particularly from about 20 ppm to about 150 ppm of theworking electrolyte.

Suitable depolarizers include nitroaromatic compounds, such as2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid,3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroace tophenone,3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole,3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde,3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol,3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid,3-nitrophthalic acid, 4-nitrophthalic acid. Particularly suitablenitroaromatic depolarizers for use in the present capacitor arenitrobenzoic acids, anhydrides or salts thereof, substituted with one ormore alkyl groups (e.g., methyl, ethyl, propyl, butyl). Specificexamples of such alkyl-substituted nitrobenzoic compounds include, forexample, 2-methyl-3-nitrobenzoic acid, 2-methyl-6-nitrobenzoic acid,3-methyl-2-nitrobenzoic acid, 3-methyl-4-nitrobenzoic acid,3-methyl-6-nitrobenzoic acid, 4-methyl-3-nitrobenzoic acid, anhydridesor salts thereof.

Specific examples of suitable working electrolytes are described in U.S.Pat. Nos. 5,369,547 and 6,594,140, both to Evans et al., U.S. Pat. No.6,219,222, now Reissue Pat. No. Re47,435 to Shah et al., and U.S. Pat.No. 6,687,117 to Liu et al. The Evans et al. patents are incorporatedherein by reference. The Shah et al. and Liu et al. patents are assignedto the assignee of the present invention and incorporated herein byreference.

Referring back to FIG. 1, the feedthrough 66 electrically insolates theanode terminal wire 64 from the casing 12. The anode terminal wire 64extends from inside the casing 12 to the outside thereof. A hole isprovided in the surrounding sidewall 20 of the casing member 14 intowhich the feedthrough 66 is mounted. The feedthrough 66 may, forexample, be a glass-to-metal seal (“GTMS”) that contains a ferrule withan internal cylindrical bore of a constant inside diameter. Aninsulative glass provides a hermetic seal between the bore of theferrule and anode terminal wire 64 passing therethrough.

After welding the feedthrough to the casing member 14 and then sealingthe casing members 14, 16 together to form the casing 12 housing theanode and the cathode, the working electrolyte is introduced into thecasing through the fill-port 82. Filling may be accomplished by placingthe casing 12 in a vacuum chamber so that the fill-port 82 extends intoa reservoir of the working electrolyte. When the chamber is evacuated,pressure is reduced inside the casing 12. When the vacuum is released,pressure inside the casing 12 re-equilibrates, and the workingelectrolyte is drawn through the fill-port 82 into the casing 12. Thefill port 82 is then hermetically sealed, for example by welding aclosure system into the fill port to complete manufacturing thecapacitor 10.

Regardless of its configuration, the capacitor 10 of the presentinvention exhibits excellent electrical properties. For example, thecapacitor may exhibit a high volumetric efficiency, such as from about50,000 μF*V/cm³ to about 300,000 μF*V/cm³, more particularly from about60,000 μF*V/cm³ to about 200,000 μF*V/cm³, and still more particularly,from about 80,000 μF*V/cm³ to about 150,000 μF*V/cm³, determined at afrequency of 120 Hz and at room temperature (e.g., 25° C.). Volumetricefficiency is determined by multiplying the formation voltage of ananode by its capacitance, and then dividing by the product by the volumeof the anode. For example, a formation voltage may be 360 volts for ananode having a capacitance of 785 μF, which results in an anode having204,000 μF*V. If the anode occupies a volume of about 2.33 cm³, thisresults in a volumetric efficiency of about 87,596 μF*V/cm³. The anodes54, 58 and 60 of the present invention are characterized as having beenformed to a voltage that ranges from 270 volts to about 430 volts.

As previously discussed, U.S. Pat. No. 10,290,430 to Djebara et al.describes a tantalum powder having a specific charge that ranges fromabout 15,000 μF*V/g to about 45,000 μF*V/g. The prior art powder isuseful as an anode in a wet tantalum electrolytic capacitor that canoperate at a voltage that ranges from about 100 volts to about 300volts. In contrast, the tantalum powder of the present invention has alower specific charge that ranges from about 11,000 μF*V/g to about14,000 μF*V/g. Even though the specific charge of the present tantalumpowder is lower than that described in the prior art '430 patent, theenergy density of the powder is higher because the voltage term issquared in the equation E=½CV². This results in a wet tantalumelectrolytic capacitor that can operate at voltage that ranges from 250volts to 390 volts, with the tantalum anodes having a breakdown voltagethat ranges from about 250 volts to about 450 volts.

Thus, the capacitor 10 of the present invention exhibits a relativelyhigh energy density that enables it to be suitable for use in high pulseapplications. Energy density is generally determined according to theequation E=½*CV², where C is the capacitance in farads (F) and V is theworking voltage of capacitor in volts (V). The capacitance may bemeasured using a capacitance meter (e.g., Quadtech 7400 Precision LCRmeter with Kelvin Leads, 1-volt bias and 1-volt signal) at operatingfrequencies that range from 10 to 120 Hz (e.g., 120 Hz) at a temperatureof 25° C.

For example, the capacitor 10 may exhibit an energy density of about 2.0joules per cubic centimeter (J/cm³), particularly about 3.0 J/cm³, andmore particularly from about 3.5 J/cm³ to about 12.0 J/cm³, and stillmore particularly from about 6.0 J/cm³ to about 9.0 J/cm³. The capacitor10 may also exhibit a relatively high “breakdown voltage”, defined asthe voltage at which the capacitor fails, such as about 180 volts ormore, more particularly 200 volts or more, and still more particularlyfrom about 250 volts to about 450 volts.

The equivalent series resistance (“ESR”), which is the extent to whichthe capacitor 10 acts like a resistor when charging and discharging inan electronic circuit, is preferably less than about 15 ohms,particularly less than about 10 ohms, more particularly less than about5 ohms, and still more particularly from about 0.5 ohms to about 4.5ohms, as measured with a 1-volt bias and 1-volt signal at a frequency of120 Hz.

In addition, the leakage current, which generally refers to the currentflowing from one conductor to an adjacent conductor through aninsulator, can be maintained at relatively low levels. For example, thenumerical value of the normalized leakage current of the capacitor 10 ofthe present invention is less than about 10 nA/μF*V, more particularlyless than about 5 nA/μF*V, and still more particularly less than about 2nA/μF*V, where nA is nanoamps and μF*V is the product of the capacitanceand the rated voltage. Leakage current may be measured holding thecapacitor 10 at a certain rated voltage after a charging time of fromabout 60 to about 300 seconds and measuring current.

The electrolytic capacitor 10 of the present invention is useful invarious applications, including but not limited to implantable medicaldevices, such as implantable defibrillators, subcutaneous implantabledefibrillators, pacemakers, cardioverters, neural stimulators, drugadministering devices, etc. In one embodiment, for example, thecapacitor 10 may be employed in an implantable medical device configuredto provide a therapeutic high voltage (e.g., between approximately 500volts and approximately 850 volts, or, desirably, between approximately600 Volts and approximately 900 volts) treatment for a patient. Themedical device has a housing that is hermetically sealed andbiologically inert. One or more leads are electrically coupled betweenthe medical device and the patient's heart via a vein. Cardiacelectrodes are provided to sense cardiac activity and/or provide avoltage to the heart. The medical device, for example the implantablecardioverter defibrillator (“ICD”) also contains a capacitor bank thattypically contains two or more capacitors 10 connected in series andcoupled to a battery. The battery supplies energy to the capacitor bank.Due in part to high conductivity, the capacitor 10 of the presentinvention can achieve excellent electrical properties and thus issuitable for use in the capacitor bank of an ICD.

The present invention may be better understood by reference to thefollowing example.

Test Procedures

Capacitance (“CAP”), equivalent series resistance (“ESR”) and leakagecurrent (“DCL”) were tested in an aqueous neutral electrolyte at atemperature of 37° C.±0.5° C.

Capacitance (“CAP”)

Capacitance was measured using a Quadtech 7400 Precision LCR meter withKelvin Leads with 1-volt DC bias and a 0.5-volt peak to peak sinusoidalsignal. The operating frequency was 120 Hz.

Equivalent Series Resistance (“ESR”)

Equivalence series resistance was measured using a QUADTECH 7400Precision LCR meter with Kelvin Leads 1-volt DC bias and a 0.5-volt peakto peak sinusoidal signal. The operating frequency was 120 Hz.

Leakage Current (“DCL”)

Leakage current was determined by charging to 250V for 300 secondswithout any resistor in series.

Example

Anodes were formed from a nodular, magnesium-reduced tantalum powder(H.C. Starck, now Taniobis GmbH). The powder was pressed to 5.5 g/cm³density. The tantalum pellets were vacuum sintered at 1,500° C. for 20minutes. Upon sintering, the pellets were anodized in a solution (seetable below) containing PEG/water or EG/water with phosphoric acid at atemperature of 40° C. and a conductivity of 1 mS/cm. The anodes werethen joined together with two cathodes prepared from ruthenium oxidecoated titanium sheets (0.1 mm thick) separated with microporousseparator material. The resulting capacitors were then tested asdescribed above. The results are set forth below.

Initial Test Results and Discussion

Anodes were formed and tested at 390V, 360V, 300V, 282V and 250V for thereceived lots. The table below summarizes the overall results and isplotted on FIGS. 7, 8 and 9. The energy variation in the 360V resultsfor FIG. 4 are related to changes in anode sintering conditions.

For the tantalum material of the present invention, the target anodicenergy density is approximately 11 J/cc, and the DCL should be less than1 nA/uFV.

Summary of Electrical Test Results

Powder Form Form Test Form AVG Lot # Anodes Voltage Electrolyte VoltageFailures AVG J/cc nA/μFV A 4 270 24% PEG400 250 1 11.3 0.94 A 3 428 66%EG 390 0 9.69 5.02 A 2 390 66% EG 360 0 10.95 2.75 B 5 390 66% EG 360 010.74 1.26 A 5 390 66% EG 360 1 10.27 3.14 A 3 330 24% PEG400 300 3 NANA A 3 330 55% PEG400 300 3 NA NA A 3 330 66% EG 300 0 10.51 2.49 B 4310 24% PEG400 282 2 11.06 7.47 B 3 310 66% EG 282 0 10.46 1.8 B 3 39066% EG 360 0 11.31 1.16

The nodular tantalum powder achieved a relatively high capacitance andlow DCL. Using the capacitance values obtained and assuming an operatingvoltage, energy density (E=0.5*CV²) indicated the nodular tantalumpowder also had a significantly higher energy density.

Although several embodiments of the present invention have beendescribed in detail, that is for purposes of illustration. Variousmodifications of each embodiment may be made without departing from thescope of the present invention. Accordingly, the present invention isnot to be limited, except as by the appended claims.

What is claimed is:
 1. An electrolytic capacitor for use in an implantable medical device, the electrolytic capacitor comprising: a) a casing; b) at least one tantalum anode housed in the casing, the tantalum anode comprising an anodically oxidized pellet formed from a pressed and sintered tantalum powder, wherein the tantalum powder has a specific charge that range from about 11,000 μF*V/g to about 14,000 μF*V/g; c) a leadwire that extends from the tantalum anode to outside the casing, the lead wire being electrically isolated from the casing; d) a cathode comprising a cathode material coated on at least one inner surface of the casing or coated on a conductive substrate electrically connected to the casing, the cathode material facing the anode; and e) a fluidic working electrolyte contained in the casing in contact with the anode and the cathode.
 2. The capacitor of claim 1, wherein the at least one tantalum anode has a breakdown voltage that range from about 250 volts to about 450 volts.
 3. The capacitor of claim 1, wherein the at least one tantalum anode is characterized as having been formed to a voltage that ranges from 270 volts to about 430 volts.
 4. The capacitor of claim 1, wherein the tantalum powder contains primary particles having an aspect ratio of about 1 to about
 4. 5. The capacitor of claim 3, wherein the primary particles are agglomerated with sinter necks between adjacent agglomerated particles, the sinter necks having a thickness that ranges from about 200 nanometers to about 1,100 nanometers.
 6. The capacitor of claim 1, wherein the tantalum powder has an energy density that ranges from about 2.0 J/cm³ to about 11.0 J/cm³.
 7. The capacitor of claim 1, wherein the casing houses at least two tantalum anodes with a cathode material contacted to a cathode current collector positioned between the anodes.
 8. The capacitor of claim 1, wherein the tantalum powder is formed by reacting an oxide of tantalum with a reducing agent that contains magnesium, strontium, barium, cesium, calcium, aluminum, and a combination thereof.
 9. The capacitor of claim 1, wherein the tantalum powder has a specific surface area that ranges from about 0.4 m²/g to about 1 m²/g.
 10. The capacitor of claim 1, wherein the tantalum powder has no more than about 50 ppm of an alkali metal.
 11. The capacitor of claim 1, wherein the tantalum powder is nodular or angular.
 11. The capacitor of claim 1, wherein the primary particles of tantalum have a median size that ranges from about 10 nanometers to about 500 nanometers.
 13. The capacitor of claim 1, wherein the at least one tantalum anode has a thickness that ranges from about 0.1 millimeters to about 3.5 millimeters, and a width that ranges from about 2 to about 25 millimeters.
 14. The capacitor of claim 1, wherein the casing is selected from the group of titanium, tantalum, nickel, molybdenum, niobium, cobalt, stainless steel, tungsten, platinum, palladium, silver, copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron, and mixtures and alloys thereof.
 15. The capacitor of claim 1, wherein the cathode material is selected from an anodized-etched conductive material, a sintered active material with or without oxide contacted thereto, a double layer capacitive material, a finely divided carbonaceous material, graphite, carbon, platinum black, a redox, pseudocapacitive, an under potential material, an electroactive conducting polymer, polyaniline, polypyrrole, polythiophene, polyacetylene.
 16. The capacitor of claim 1, wherein the cathode material is selected from an oxide of a metal, a nitride of the metal, a carbon nitride of the metal, and a carbide of the metal, the metal being selected from the group of ruthenium, cobalt, manganese, molybdenum, tungsten, tantalum, iron, niobium, iridium, titanium, zirconium, hafnium, rhodium, vanadium, osmium, palladium, platinum, nickel, and lead.
 17. The capacitor of claim 1, wherein the working electrolyte has a pH that ranges from about 5.0 to about 7.5.
 18. The capacitor of claim 1, wherein a separator is positioned between the anode and the cathode.
 19. The capacitor of claim 1, wherein the casing comprises a first casing member having a face wall meeting a surrounding sidewall that extends to an edge, and a second plate-shaped casing member that is sealed to the edge of the surrounding sidewall of the first casing member.
 20. The capacitor of claim 1, wherein the casing comprises mating clamshell-type casing member that are hermetically sealed together.
 21. An implantable medical device comprising the capacitor of claim
 1. 22. An electrolytic capacitor for use in an implantable medical device, the electrolytic capacitor comprising: a) a casing; b) at least one tantalum anode housed in the casing, the tantalum anode comprising an anodically oxidized pellet formed from a pressed and sintered tantalum powder, wherein the tantalum powder has a specific charge that ranges from about 11,000 μF*V/g to about 14,000 μF*V/g; c) a leadwire that extends from the tantalum anode to outside the casing, the lead wire being electrically isolated from the casing; d) a cathode comprising ruthenium oxide coated on at least one inner surface of the casing or coated on a conductive substrate electrically connected to the casing, the ruthenium oxide facing the anode; and e) a fluidic working electrolyte contained in the casing in contact with the anode and the cathode. 